Natural deduction provides a method to demonstrate that a reasoning is correct, but, how can you prove that a reasoning is non-correct? It can't be done with natural deduction.
We are in this situation: we have sequent , and we think that there exists a model (set of values) which make true -gamma- but not . Well, then we just have to find it to prove that the sequent is invalid. This model is called countermodel, and we can find it in several ways. I think that the simplest one is intuitively: start trying different values which we regard as possible countermodel, until we find a good one.
For instance, is invalid (), since when is true and is false, the left part (antecedent) becomes true but the right part (consequent) is false, so is not a consequence of that from the left part.
Daniel Clemente Laboreo 2005-05-17